1. Field of the Invention
This invention concerns mechanical creping of wet-laid or dry form (air-laid) webs. More particularly this invention concerns controlling the mechanical creping process by employing two creping adhesives having properties different then each other.
2. Description of the Prior Art
Paper webs are conventionally softened by working them in different ways such as by mechanically creping them from a surface, usually a drying surface such as a Yankee dryer with a creping doctor blade. Such a process disrupts and breaks many of the inter-fiber papermaking bonds in the paper web which are formed during drying. These inter-fiber bonds are the principle source of strength in ordinary paper since very little strength results from the physical entanglement of the fibers.
Creping adhesives have been employed for adhering the webs to the creping surface sometimes in combination with release agents in order to control the degree of adhesion between the web and the creping surface. Control of the adhesion permits the continuous production of creped webs having substantially uniform characteristics imparted by the creping process.
Creping adhesives have also been used for adhering relatively dry paper webs to a creping surface since such dry webs do not have the usual natural adherence obtained by pressing wet webs to a creping surface.
The softness of paper webs has been increased by chemically impeding or preventing the inter-fiber papermaking bonds with a chemical debonder which also tends to weaken the sheet. Usually a wet or dry strength chemical is added to the sheet to make up for the strength loss caused by the chemical debonder. Unfortunately, the chemical debonders also tend to interfere with proper adhesion of the fibrous webs to a creping surface which tends to prevent creping of a chemically debonded web.
Creping adhesives have been added to the paper sheet or directly on to the creping surface to overcome the effect of the chemical debonder and obtain proper adhesion between the chemically debonded fibrous sheet and the creping surface.
The nature of the finished paper depends upon the mechanical forces at the locus of removal of the web from the cylinder, and also upon the angle of removal. Without a doctor blade or with a doctor blade substantially tangent to the circumference of the cylinder, the paper tends to have a smoother, "machine-glazed" finish. If the doctor blade is at an angle to the tangent, the finished paper is creped and this procedure is known as "creping off the Yankee drier."
In spite of the many desirable advantages which accrue in creping from Yankee driers, there is often much difficulty in maintaining smooth removal of the web by the doctor blade. This operation requires a delicate balance between the adhesive forces holding the web uniformly on the cylinder surface and the releasing forces occurring at the source of contact with the doctor blade. The creping forces can get out of balance. Too great of an adhesive force can result in "pickouts" of paper, or fibers remaining with the dryer surface and the inability of the cleaning doctor to remove sufficient adhesive from the dryer to avoid buildup of adhesive and an irregular surface. On the other hand, too little of an adhesive force can result in the paper being removed from the creping surface without sufficient creping action being applied to the web. Many attempts have been made to attain this needed balance but none have been entirely satisfactory. The present invention provides the desired control and balance.